Wastewater as well as un-altered waters characterized by high salt concentrations (>1,000 mg/L) are generally referred to as saline water. Highly saline wastewaters are generated in an array of industrial applications including during the exploration for and production of oil and natural gas resources from subsurface formations. Management of produced waters from oil and gas production is receiving growing scrutiny as the volumes that are now being generated in the United States alone are approaching 20 to 30 billion barrels per year. Produced waters can have total dissolved solids concentrations ranging from thousands to several hundred thousand mg/L. Two common methods of managing such brine waters are deep-well injection and evaporation ponds. Both of these approaches are disposal techniques and not considered as treatment processes nor are they beneficial uses.
The rapid growth of the world's population continues to place stress on our finite and limited freshwater resources. This stress has led stakeholders to seek out ways to reduce the amounts of freshwater that are used in industrial and agricultural systems. Agricultural activities are particularly targeted because they generally represent the largest consumer of freshwater in many states. For example crop irrigation is the single largest use (39% of total) of fresh water in the United States. To meet this challenge many are turning to the reuse of ‘impaired’ waters or waters naturally enriched with salts for irrigation of non-food based crops and vegetation. The paradigm has shifted from considering these effluent streams as ‘waste’ to now seeing them as a valuable resource. Realizing the possibility of reusing brine flows for some beneficial purpose requires that the existing challenges associated with desalination processes be overcome and overcome in a manner that is cost effective and practically feasible.
Along these lines, a number of water purification processes are utilized. Processes for the purification of water are sometimes classified as filtration, distillation or osmosis. In filtration processes, impurities in particulate form are removed using porous constructions or filters. In cases where very small particles must be filtered, polymer membranes are used which are microporous, that is, the membranes have very small holes through which the particulates to be filtered cannot pass. However, filtration processes are not utilized for desalination.
Aqueous solutions containing dissolved salts (e.g., brine streams) are usually purified (desalinated) by ion exchange, pressure driven membrane processes (e.g., nano-filtration, reverse osmosis) non-pressure driven membrane processes (forward osmosis, membrane distillation), mechanical distillation, and crystallizers. While effective, these technologies are highly energy intensive and require extensive infrastructure.
Another process for desalinating water is pervaporation. Pervaporation (or pervaporative separation) is a processing method for the separation of mixtures of liquids by partial vaporization through a non-porous membrane. This process can be non-pressure driven and therefore requires significantly less energy input than the prior discussed technologies. Pervaporation membranes separate aqueous solutions based on differing rates of diffusion and solubility into a typically non-porous membrane, followed by an evaporative phase change. Water transport across a pervaporation membrane occurs in three steps—attachment to the membrane surface on the interior side (i.e., feed side) of the membrane, followed by diffusion into and across the membrane and a final step in which permeate (vapor) desorbs from the opposing side (e.g., permeate side) of the membrane. The primary driving force for pervaporation is the partial pressure difference of the permeating component between the feed and permeate sides of the membrane.